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FIR Halfband Decimator

Decimate signal using polyphase FIR halfband filter

Library

Filtering/Filter Designs

dspfdesign

Description

The FIR Halfband Decimator block performs polyphase
decimation of the input signal by a factor of two. The block uses
an FIR equiripple design to construct the halfband filters. The implementation
takes advantage of the zero-valued coefficients of the FIR halfband
filter, making one of the polyphase branches a delay. You can also
use the block to implement the analysis portion of a two-band filter
bank to separate a signal into lowpass and highpass subbands.

The input signal can be a real- or complex-valued column vector
or matrix. If the input signal is a matrix, each column of the matrix
is treated as an independent channel. The number of rows in the input
signal must be a multiple of 2. The block supports fixed-point operations
and ARM® Cortex® code generation. For more information on ARM Cortex code
generation, refer ARM Cortex-M and ARM Cortex-A Optimization.

Dialog Box

Main Tab

Filter
specification

Parameters used to design the FIR halfband filter. Because the
filter design has only two degrees of freedom, you can specify only
two of three parameters:

Transition width and stopband attenuation
(default) — Design the filter using Transition width
(Hz) and Stopband attenuation (dB).
This design is the minimum order design.

Filter order and transition width —
Design the filter using Filter order and Transition
width (Hz).

Filter order and stopband attenuation —
Design the filter using Filter order and Stopband
attenuation (dB).

Transition
width (Hz)

Transition width, specified as a real positive scalar in Hz.
The transition width must be less than half the input sample rate.
You can specify the transition width when Filter specification is
set to Filter order and transition width or Transition
width and stopband attenuation. The default is 4.1e3.

Filter
order

Filter order, specified as an even positive integer. You can
specify the filter order when Filter specification is
set to Filter order and transition width or Filter
order and stopband attenuation. The default is 52.

Stopband
attenuation (dB)

Stopband attenuation, specified as a real positive scalar in
dB. You can specify the stopband attenuation when Filter
specification is set to Filter order and stopband
attenuation or Transition width and stopband
attenuation. The default is 80.

Output highpass subband

When you select this check box, the block acts as a synthesis
filter bank and synthesizes a signal from the highpass and lowpass
subbands. When you clear this check box, the block acts as an FIR
halfband decimator and accepts a single vector– or matrix–valued
input.

Inherit
sample rate from input

When you select this check box, the block inherits its sample
rate from the input signal. When you clear this check box, you specify
the sample rate in Input sample rate (Hz).

Input
sample rate (Hz)

Input sample rate, specified as a scalar in Hz. The default
is 44100.

View Filter Response

Opens the Filter Visualization Tool FVTool and displays the
magnitude/phase response of the FIR Halfband Decimator.
The response is based on the block dialog box parameters. Changes
made to these parameters update FVTool.

To update the magnitude response while FVTool is running, modify
the dialog box parameters and click Apply.

Simulate using

Type of simulation to run. You can set this parameter to:

Code generation (default)

Simulate model using generated C code. The first time you run
a simulation, Simulink® generates C code for the block. The C
code is reused for subsequent simulations, as long as the model does
not change. This option requires additional startup time but provides
faster simulation speed than Interpreted execution.

Interpreted execution

Simulate model using the MATLAB® interpreter. This
option shortens startup time but has slower simulation speed than Code
generation.

Data Types Tab

Rounding mode

Rounding method for the output fixed-point operations. The rounding
methods are Ceiling, Convergent, Floor, Nearest, Round, Simplest,
and Zero. The default is Floor.

Coefficients
data type

Fixed-point data type of the coefficients, specified as one
of the following:

fixdt(1,16) (default)
— Signed fixed-point data type of word length 16,
with binary point scaling. The block determines the fraction length
automatically from the coefficient values in such a way that the coefficients
occupy maximum representable range without overflowing.

fixdt(1,16,0) —
Signed fixed-point data type of word length 16 and
fraction length, 0. You can change the fraction
length to any other integer value.

<data type expression> —
Specify the data type using an expression that evaluates to a data
type object, for example, numeric type (fixdt,[
],16, 15). Specify
the sign mode of this data type as [ ] or true.

Refresh Data Type — Refreshes
to the default data type.

Click the Show data type assistant button to display the data type
assistant, which helps you set the stage input parameter.